Sibling autism risk 'high'

16th August 2011

In about 20% of cases, autism spectrum disorders (ASDs) are shared by siblings, according to a recent US study.

Sally Ozonoff, a psychiatry professor with the Mind Institute at the University of California at Davis, said that she and her colleagues were all a bit surprised by how frequently siblings shared autistic traits.

Experts are not certain if genetics and upbringing might play equal roles in the development of ASDs, and do not understand what might cause autistic traits to develop.

Some people have proposed that infections and pollution might both play a role, and as regards the recent study, the fact that siblings are likely to be exposed to the same influences could also be a factor.

ASDs are also much more frequently diagnosed in boys than in girls, and Ozonoff said that she believed the risk factors could be particular individual families.

For the study, the researchers enrolled 664 infant study subjects from 12 different sites in the US and Canada, none of whom showed any signs of developing an ASD.

Each of the infants had at least one older brother or sister with autism.

At the end of the study, the researchers counted 132 children who went on to develop autistic traits by age 3.

Previous studies had shown a much lower rate of prevalence, some as little as 3%.

The rate was highest among infants who had more than one older sibling with autism, and 32% of all such infants went on to develop autism.

Catherine Lord, a director at the New York-Presbyterian Medical Centre, who was not involved in the study, said that the recent study would help parents who already had autistic children.

She said the finding had implications for decisions about how many children to have, in such families.

Ozonoff said the study should prompt families and their children's doctors to be vigilant, in families where autism was already recorded.

Alycia Halladay, a research director at the advocacy group Autism Speaks, said the study's prevalence estimate was more accurate than previous studies.

She said the study strengthened the idea that family history was a risk factor.

Ozonoff said that people who worked with autism and ASDs knew the rate of occurrence was a lot higher than previously believed.

A Korean study earlier this year showed that as many as one third of all children may fit the diagnosis of autism.